Some knotty FOIA debates end up generating confusing and apparently contradictory case law; some others get resolved by an authoritative three-person Upper Tribunal. The recent judgment of the UT (Mrs Justice Farbey and UT Judges Mullan and Wikeley) in FCDO v IC, Williams and Others  UKUT 248 (AAC) is a neat example of the latter. It deals with the interplay between sections 23 and 24 of FOIA.
The problem, from a public authority’s perspective: the absolute exemption under section 23 of FOIA (information supplied by/relating to security bodies) is intended to afford the widest possible protection, and to block any backdoor inferential insights into the involvement of the security bodies, so as to avoid prejudicing their work in the interests of national security. But there is of course also a national security exemption (section 24 of FOIA). So if in some cases the public authority refuses a request under section 24 only, but in other cases refuses under section 23 only, there is ample scope for backdoor inferences about when security bodies were or were not involved.
The solution, from a public authority’s perspective: refuse the request, citing both section 23 and 24, even though you know full well which one you actually rely on.
The challenge, from a requester’s perspective: that sort of masking isn’t allowed, because section 17 FOIA requires the public authority to ‘specify the exemption’ on which it relies when refusing a request. So, for example, if you mean section 23, say so (after all, you don’t need to say which security body or how they are involved), and if you don’t, don’t.
The resolution: public authorities can lawfully run sections 23 and 24 together, without saying whether or not section 23 in fact applies to a particular request, “so as to protect the interests of national security by masking whether or not the information requested relates to one of the security bodies listed in section 23(3)” (at ). The decision is here: FCO IC Williams
The UT applied the leading cases on statutory interpretation, such as R (Quintavalle) v Secretary of State for Health  UKHL 13,  AC 687 per Lord Bingham at :
“The court’s task, within the permissible bounds of interpretation, is to give effect to Parliament’s purpose. So the controversial provisions should be read in the context of the statute as a whole, and the statute as a whole should be read in the historical context of the situation which led to its enactment…”
To similar, effect, see also R (Fylde Coast Farms Ltd) v Fylde BC  UKSC 18,  1 WLR 2794 per Lord Sales at :
“Even where particular words used in a statute appear at first sight to have an apparently clear and unambiguous meaning, it is always necessary to resolve differences of interpretation by setting the particular provision in its context as part of the relevant statutory framework, by having due regard to the historical context in which the relevant enactment came to be made and, to the extent that its purpose can be identified (which may require examination of admissible travaux preparatoires), to arrive at an interpretation which serves, rather than frustrates, that purpose.”
On that approach, the duty to “specify” an exemption is fulfilled by citing sections 23 and 24 in the alternative, taking into account in particular the nature and purposes of those statutory exemptions (see the UT’s summary of the established principles at  and ).
Christopher Knight acted for the Information Commissioner, whose analysis – chiming with that of the FCDO – was accepted by the UT.